Problem: In a triangle with integer side lengths, one side is three times as long as a second  side, and the length of the third side is 15. What is the greatest possible perimeter of the triangle?
Answer: Let the sides of the triangle have lengths $x$, $3x$, and 15. The Triangle Inequality implies that $3x<x+15$, so $x<7.5$. Because $x$ is an integer, the greatest possible perimeter is $7+21+15=\boxed{43}$.